Most solar stills, and particularly those of the greenhouse type, are of a low thermal efficiency because of a loss of heat. In such systems, the vapor produced in the still is condensed on the surface of glass panels, giving up the heat of condensation to the atmosphere, causing thereby a loss of energy. The purpose of the present invention is to improve the thermal efficiency of solar stills by using the heat of condensation for preheating the distilland before evaporating it.